Adoption installs the master wherever the adopted clone already is, so
dsh-upgrade and dsh-customize can no longer state <source>/master as fact.
Both skills already derive the master from the launcher, so the procedures
hold; only the layout description was wrong. dsh-upgrade now names
`git rev-parse --git-common-dir` as the way to resolve it.
The legacy-migration clauses stay: installs made before this change can
still link PATH straight at a worktree.
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